Modern technical solutions are characterized by an increased interrelatedness between heterogeneous actors and knowledge fields. No single firm can keep pace with the development of all relevant technologies. Therefore, firms must have access to external knowledge sources. Informal networks or cooperative environments are an organizational device for external learning. In the paper the master-equation approach is applied to model the evolution of an informal network. Using this method we can totally dispense with the rigorous assumption of perfect rationality that is usually employed when modelling cooperative behaviour. Our results show that under a regime where technology is of major importance, know-how sharing and the emergence of informal networks is possible via self-organization (C) 1997 Elsevier Science Ltd.